Display system using variable frequency, constant amplitude, constant width pulses applied to a cathode ray tube



Oct. 4, 1966 P. M. MOSER ET AL DISPLAY SYSTEM USING VARIABLE FREQUENCY, CONSTANT AMPLITUDE, CONSTANT WIDTH PULSES APPLIED TO A CATHODE RAY TUBE Filed July 51, 1963 2o 22 2s 24 I P f Y VIDEO PHASE INPUT SPLITTER M'XER v.c.o. AMP.

Mv 8 SCHMIDT F j 8t TRIGGER VIDEO AMP.

i TO VIDEO AMP. (11) INVENTORS PAUL M. MOSER BY THEODORE R. TRILLING TO DIFFERENTIATOR CIRCUIT ATTORNEY United States Patent DISPLAY SYSTEM USING VARIABLE FRE- QUENCY, CONSTANT AMPLITUDE, CON- STANT WIDTH PULSES APPLIED TO A CATHODE RAY TUBE Patti M. Moser, Abington, and Theodore R. Trilling,

Levittown, Pan, assignors to the United States of Ameriea as represented by the Secretary of the Navy Filed July 31, 1963, Ser. No. 299,126 Claims. (Cl. 315-30) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a display system and more particularly to a frequency intensity modulation display system.

In display systems it is often desirable to visually represent certain types of information as varying degrees of intensity or tonal shades on the face of a display device such as a cathode ray tube. Where the information is to be displayed in the form of differences in intensity or tonal shade, the total number of shades of which a display device is capable of presenting to a viewer becomes important.

It is well known that the total number of tonal shades capable of being represented on the face of a cathode ray tube or silimar display device is limited by the non linearity of the transfer function of the display device. Thus, a cathode ray tube without other modifications produces only six to eight shade variations in picture-type information.

The transfer function of a typical cathode ray tube is given as l K y, where I is the luminous intensity, 2 the drive voltage above cut-off and K and 'y are constants over the useful range of e. The exponent 'y is called simply the gamma and usually has a value between 2 and 2.4 (2.2 being average). The non-linearity of the transfer function of the cathode ray tube is at tributed to the exponent 'y.

A method of linearizing the transfer function of a cathode ray tube to thereby increase the total number of shades or intensities capable of being produced on the face of a cathode ray tube is called gamma correction of the video information.

One method of gamma correction is to pass the video signal through a pre-distortion device before driving the cathode ray tube. The pro-distortion device distorts the video signal in such a manner as to neutralize the effect of the non-linearity of the transfer function of a cathode ray tube to thereby effectively linear-ize the transfer function and permit a large number of gray shades to be produced on the face of the cathode ray tube. Such a method entails substantial modification to the basic display system and is limited to the cathode ray tube having the gamma for which it is developed.

The present invention contemplates a system of displaying intensity information on a cathode ray tube which linearizes the cathode ray tube itself thereby eliminating the necessity of gamma correction of the video signal. This system involves the trading of spatial resolution of the cathode ray tube to obtain greater tonal resolution. Since cathode ray tubes are now available with spatial resolutions better than 1000 lines per inch, considerable spatial resolution is available for trading in display systems requiring low spatial resolutions. The technique of linearizing the cathode ray tube by exchanging spatial resolutions for tonal resolution contemplates the use of frequency intensity modulation. The system of the present invention uses pulse symmetry modulation or duty cycle modulation to change the average intensity of the cathode ray tube within each picture element, When the spatial resolution of the cathode ray tube is not exceeded, the frequency intensity modulation technique of the present invention causes the gamma of the transfer function of the cathode ray tube to approach unity.

If each picture resolution element on a cathode ray tube display is composed of spots of uniform intensity spaced at intervals inversely proportional to the video signal amplitude and if the display is viewed from the proper distance, it will appear to be intensity modulated. The scan lines are then made up of picture elements each of which comprises sub-resolution elements consisting of a bright spot-dark spot pair. The display intensity if observed from the proper distance will appear to be the average intensity of each sub-resolution element. Changing the bright spot population density within the picture element produces tonal changes.

The present invention contemplates a form of subresolution population density modulation, that is, frequency intensity modulation. In this system the size of the sub-resolution elements remains constant and the tonal shades are produced by varying the ratio of beam-on time to beam-off time. In the contemplated form of the present invention the beam-on time or light spot size is held constant and the size of the sub-resolution elements or period is changed to vary the beam-on to the beamoff ratio.

In a preferred embodiment the video information in the form of linear amplitude information is coded into linear frequency changes which are transferred through the cathode ray tube as digital information requiring only two amplitude states. The cathode ray tube translates the digital information into spatial digital intensity information which is averaged by the eye back into amplitude intensity modulation. The information is transferred linearly through the non-linear elements of the system as long as it remains digital and no averaging is allowed to take place in the system before all the linear elements are passed. By operating on the information in digital form, the frequency coded information can be amplified accurately by pulse amplifiers and the frequencies may range from a maximum down to zero frequency. Because the information is digital in nature on a cathode ray tube it is averaged by the eye, the Whole process is equivalent to the half-tone process of photo-engraving.

By use of the present invention, limitations of display devices such as cathode ray tubes whose input-output functions are not linear and whose tonal response is limited by time varying noise are overcome.

Therefore, it is an object of the present invention to provide a display system having the capability of displaying a wide range of tonal shades.

Another object of the present invention is to provide a display system utilizing frequency intensity modulation whereby improvement in tonal resolution is gained at the expense only of unnecessary spatial resolution.

A further object of the present invention is to provide a display system wherein the non-linear transfer function of the display device is compensated to provide a wider range of tonal response in the display.

A still further object of the present invention is to provide a display system utilizing a system of frequency intensity modulation which is the electronic counterpart of the half-tone process of photo-engraving.

Yet another object of the present invention is to provide a display system wherein the non-linear transfer function of the display device is linearized without resort to complicated gamma correction of the display system.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 illustrates in block diagram form a preferred embodiment of the present invention.

FIG. 2 illustrates in detailed schematic form the monostable multivibrator of FIG. 1.

Referring more particularly to FIG. 1, linear frequency modulator 11 receives an input of the video information to be displayed from a video signal source here represented by block 20. Linear frequency modulator 11 converts the video signal into a frequency modulated wave. Linear frequency modulator 11 which is more fully described hereinbelow, is a linear voltage to frequency converter operable over a range of zero to 1 megacycle per second. In linear frequency modulator 11 the video signal input is used to modulate alinear frequency to produce a signal with a frequency varying in accordance with the amplitude of the video signal input.

The output from linear frequency modulator 11 is amplified in amplifier 12 greatly in excess of the linear range of amplifier 12 to square off the sine waves produced in the output of linear frequency modulator 11. The output of amplifier 12 is fed to Schmidt trigger circuit 13 which provides as an output variable frequency square waves with sharp rise and fall time. The squared off sine waves from amplifier 12 provide sharp triggering of Schmidt trigger circuit 13. The output of Schmidt trigger circuit 13 is fed to differentiator 14 wherein the square waves from Schmidt trigger circuit 13 are differentiated to form trigger pulses. The output pulses from ditferentiator circuit 14 are fed as trigger pulses to monostable multivibrator 16 which provides as an output a train of pulses having constant amplitude and constant width but a variable repetition rate. Monostable vibrator 16 which forms an important part of this invention and which is discussed more fully hereinbelow provides in response to the output of differentiator 14 a train of pulses wherein each pulse is of constant width and amplitude, and has very fast rise and fall times. In addition, monostable multivibrator has a very quick recovery time. The recovery time ratio, that is, the ratio of the recovery time of the pulsed width determines the highest useable frequency or duty cycle which determines the maximum brightness (or darkness) of the display.

The pulse output from monostable multivibrator 16 is fed to video amplifier 17. Video amplifier 17 amplifies each of the pulses in the pulse train output from monostable multivibrator 16 to provide sufficient power for each of the pulses from monostable multivibrator to drive a cathode ray tube from cut-off to saturation. The gain of video amplifier 17 is relatively small but the band width must be large enough to allow the pulses to be amplified without deterioration. The output from video amplifier 17 is connected to the intensifier grid of a cathode ray tube 18.

On each line of sweep of the cathode ray tube 18 (sweep and timing circuits not being shown) the beam of cathode ray tube 18 will be turned on by each of the pulses from monostable multivibrator 16 for the duration of the pulse. Information in the form of tonal shades or intensity gradations are displayed on the face of the cathode ray tube by virtue of the variations in population density of the beam spots.

A requirement of the linear frequency modulator 11 used in the present invention is that it be capable of linear operation over a wide range of frequencies relative to the carrier frequency. During the construction of this invention it was found that if two high frequency, nonlinear voltage controlled oscillators were driven from opposite phase voltages and beat together the resulting difference signal could be made to cover the range of zero to 1 megacycle per second with very good linearity.

Referring now more particularly to the linear frequency modulator 11 of FIG. 1, there is shown voltage controlled oscillator 19 and voltage controlled oscillator 21. The

inputs of voltage controlled oscillators 19 and 21 are connected to phase splitter 22 which receives the video signal input. The output of voltage controlled oscillators 19 and 21 are connected to mixer 23 whose output is fed to amplifier 12 through the low pass filter 24.

The video signal is fed to the input of phase splitter 22 which provides two outputs identical to the input except that each output is 180 degrees out of phase with the other. Thus, the two voltage controlled oscillators 19 and 21 are driven out of phase relative to each other and provide. an input to mixer 23 wherein they are beat to produce a difference signal. This technique provides out of mixer 23 a difference signal in Whichthe normally present first distortion term has been eliminated. Low pass filter 24 eliminates unwanted frequencies from the output of linear frequency modulator 11. Utilizing the linear frequency modulator just discussed provides a signal output modulated by the video signal input which has less than 4 percent distortion at maximum deviation of plus or minus percent of the carrier frequency.

Referring now. more particularly to FIG. 2, there is shown the monostable multivibrator 16 of FIG. 1. This circuit is a non-cutoff monostable multivibrator, that is, neither of the main switching transistors are ever completely cut off. Monostable multivibrator 16 comprises transistors 26, 27 and 28. The base of transistor 26 is connected to ditferentiator circuit 14. Diodes D and D are connected to the base of transistor 26 with the polarity shown. The collectors of transistors 26 and 27 are connected to a 20 volt DC. power supply through resistors 29 and 31, respectively. The base of transistor 28 is connected directly to the collector of transistor 26 while the collector of transistor 28 is connected directly to the 20 volt D.C. power supply. The base of transistor 27 is connected to the collector of transistor 26 through capacitor 32 and diode D Resistor 33 connects the base of transistor 27 to the 20 volt DC. power supply while diode D connects the base of transistor 27 to a 7 volt source. The emitters of transistors 26 and 27 are connected to ground through resistors 34 and 36, respectively. The emitters of transistors 26 and 27 are also connected together via diodes D and D which have the polarities shown. The common point of juncture of diodes D and D is connected to ground through resistor 37.

Diode D normally holds the base of transistor 26 at a potential such that its collector current is minimum, that is, transistor 26 is essentially non-conducting but not completely cut off. Due to this, the voltage across resistor 34 is less than the voltages across resistors 37 and 36 and the emitter of transistor 26 is disconnected from resistor 37. Normally transistor 27 is conducting and diode D is forward biased connecting resistors 36 and 37 in parallel.

When the positive spike of the differentiated pulse from ditferentiator circuit 14 is fed to the base of transistor 26, diode D is turned off and transistor 26 becomes conductive. This causes resistors 34 and 37 to be connected in parallel since diode D is then biased ON.

This permits the collector current of transistor 26 to rise to its maximum value which is limited by diode D which clamps the base of transistor 26 to 7.6 volts. The falling potential of the collector voltage of transistor 26 is coupled through diode D and capacitor 32 to the base of transistor 27. Therefore, diode D is disconnected from the base of transistor 27 and the base of transistor 27 drops the same number of volts as the collector of transistor 26. As the base of transistor 27 falls in voltage, its emitter will also drop in voltage thereby disconnecting resistor 36 and the emitter of transistor 27 from resistors 34 and 37 due to the back biasing of diode D By virtue of the interaction between resistors 34, 37 and 36 and diodes D and D neither transistor 26 nor 27 is permitted to be completely out off. When the emitter of transistor 27 is disconnected from resistors 34 and 37, the emitter current of transistor 27 drops to its minimum value thereby allowing its collector potential to rise to the supply voltage. This initiates the pulse.

When the positive spike from dilferentiator circuit I14 ceases, capacitor 32 is allowed to charge through resistor 33, transistor 26 (which is conducting), and resistors 34 and 37 in their parallel combination. But when the positive spike pulse ceases, diode D is again turned on causing the base of transistor 26 to regain its original potential. Thus, when capacitor 32 charges up toward the supply voltage, the base and emitter voltage of transistor 27 follows and when the voltage across resistor 36 reaches the voltage across resistors 34 and 37, diode D turns on. Therefore, transistor 27 is again turned on and gains its original stable state. This terminates the pulse. The back biasing of diode D causes transistor 26 to turn off.

When transistor 26 becomes conductive, the collector voltage back biases diode D which causes emitter follower transistor 28 to become conductive providing a very fast, low impedance discharge circuit for capacitor 32. Thus, capacitor 32 discharges rapidly through transistor 28 and diode D or to put it in other words, capacitor 32 charges up but in a different direction than its original charging. In any event, due to the rapid charging of capacitor 32 when transistor 28 is turned on, the circuit quickly becomes stable for the next trigger pulse providing a recovery time ratio of 0116 for a pulse width of one microsecond.

The monostable multivibrator just discussed is an important component in the present invention which requires that the pulse applied at the intensifier grid of the cathode ray tube maintain constant width over wide ranges of duty cycle. Monostable multivibrator 16 has extremely rapid recovery time and provides constant width and constant amplitude pulses having fast rise and fall time. The recovery time ratio, that is, the ratio of the recovery time to the pulse width determines the highest useable frequency and therefore, duty cycle which determines the maximum brightness or darkness of which the display system is capable. Monostable multivibrator 16 achieves improvement in recovery time by combining the concept of incomplete cut off of the transistors thereby decreasing the switching time with very rapid return to the stable state provided by emitter follower transistor 28 and its associated circuitry.

Operation of the system of the present invention is relatively simple. The information to be displayed is contained in the amplitude excursions of the video signal which is the input to linear frequency modulator 11. Linear frequency modulator 11 provides an output having a frequency modulated in accordance with the amplitude variations of the video signal. This output is then shaped and monostable multivibrator produces an output comprising a train of constant width, constant amplitude pulses having a rate of repetition linear-1y proportional to the amplitude variations of the video signal. The train of pulses is then amplified and finally applied to the intensity grid of cathode ray tube 18. In response to each pulse in the train, the beam of cathode ray tube is turned on for the duration of each pulse. Thus, as the beam is swept across the face of the tube a great number of spots will appear on the tube all of equal intensity. The population density i.e., number of spots in a given segment determines the shade of gray displayed on the face of the tube. The population density is a linear function of the amplitude variations of the video input signal. Therefore, the information is displayed on the face of the tube as gradations in tonal shade. In this respect the present invention is quite similar to the halftone process normally associated with photo-engraving.

The only limitation imposed on the present invention is that overlapping of beam spot-s must not take place. If this occurs, averaging takes place in the phosphor of the tube (not by the eye), and the results are equivalent to the well known amplitude intensity modulation system for producing tonal response on a display element. If this takes place, the transfer function again becomes nonlinear and the great advantage of the present invention would be nullified.

However, the resolution capability of the cathode ray tube may be exceeded without ill effects if the phosphor recovery time is fast enough to prevent averaging in the phosphor itself. Thus, where the present invention is used with phosphors having very rapid recovery time, the above disadvantage can be substantially eliminated.

In addition to the above discussed pulse frequency modulation system the present invention may utilize pulse width modulation or pulse ratio modulation to produce the desired frequency intensity modulation.

In the pulse width modulation system the size of the sub-resolution elements does not vary and there is a constant number per picture element. 7

The pulse ratio modulation system is a combination of the pulse width modulation and pulse frequency modulation system. This system has the advantage of requiring only finite bandwidth and spatial resolution at both ends of the tonal spectrum.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a display system:

first means frequency modulating a carrier frequency in accordance with amplitude variations in an input signal,

second means including wave shaper means and multivibrator means connected to said first means generating a pulse in response to each cycle of said carrier frequency,

cathode ray tube means,

third means connecting said second means to the intensifier grid of said cathode ray tube means whereby the beam of said cathode ray tube means is turned on in response to each of said pulses.

2. In a display system:

input means providing a video information signal,

frequency modulator means connected to said input means modulating a carrier frequency in accordance with amplitude variations in said video information signal,

wave shaper means connected to said frequency modulator means providing a train of pulses having a frequency proportional to the frequency of said modulated carrier frequency,

monostable multivibrator means connected to said wave shaper means providing a pulse of constant width and amplitude in response to each pulse in said train of pulses,

cathode ray tube means,

means connecting said monostable multivibrator means to the intensifier grid of said cathode ray tube means whereby the electron beam of said cathode ray tube means is turned on in response to each of said pulses from said monostable multivibrator means.

3. In a display system according to claim 2 wherein said frequency modulator means comprises:

first oscillator means having a first operating frequency,

second oscillator means having a second operating frequency,

mixer means connected to said first and second oscillator means providing as an output a signal which is the difference frequency between said first and second operating frequencies,

phase splitter means connected to said input means and to said first and second oscillator means providing said first oscillator means with said video information signal as an input and said second oscillator means with an input signal equal in amplitude but degrees out of phase with said video information signal whereby said output signal from said mixer means is modulated in accordance with amplitude of said video information signal.

4. In a display system according to claim 2 wherein said monostable multivibrator comprises:

first transistor means being normally non-conductive,

second transistor means being normally conductive,

first circuit means connected to the emitters of said first and second transistor means preventing said first and second transistor means from ever becoming fully conducting or non-conducting,

second circuit means including a capacitor connected between the collector of said first transistor means and the base of said second transistor means switching the conductive states of said first and second transistor means when a pulse is applied to the base of said first transistor means,

first means charging said capacitor in a first direction after said first transistor means becomes conductive causing said first transistor means to become nonconductive after a predetermined time,

second means charging said capacitor in the second direction after said first transistor means is switched to the non-conductive state causing the monostable multivibrator to become stable rapidly.

5. In a display system according to claim 3 wherein said monostable multivibrator comprises:

first transistor means being normally non-conductive,

second transistor means being normally conductive,

first circuit means connected to the emitters of said first and second transistor means preventing said first and second transistor means from ever becoming fully conducting or nonconducting,

second circuit means including a capacitor connected between the collector of said first transistor means and the base of said second transistor means switching the conductive states of said first and second transistor means when a pulse is applied to the base of said first transistor means,

first means charging said capacitor in a first direction after said first transistor means becomes conductive causing said first transistor means to become nonconductive after a predetermined time,

second means charging said capacitor in the second direction after said first transistor means is switched to the non-conductive state causing the monostable multivibrator to become stable rapidly.

6. In a display system:

input means providing a video information signal,

frequency modulator means connected to said input means modulating a carrier frequency in accordance with amplitude variations in said video information signal,

amplifier means connected to said frequency modulator means squaring off the output of said frequency modulator,

trigger circuit means connected to said amplifier means converting the output of said amplifier means to a pulse train of square wave pulses having a repetition rate proportional to the frequency of the output of said amplifier means,

difierentiator circuit means connected between said amplifier means and said monostable multivibrator means providing said monostable multivibrator means with a trigger pulse for each pulse from said trigger circuit means,

monostable multivibrator means connected to said wave shaper means providing a pulse of constant width and amplitude in response to each pulse in said train of pulses,

cathode ray tube means,

means connecting said monostable multivibrator means to the intensifier grid of said cathode ray tube means whereby the electron beam of said cathode ray tube said frequency modulator means comp-rises:

first oscillator means having a first operating frequency,

second oscillator means having a second operating frequency,

mixer means connected to said first and second oscillator means providing as an output a signal which is the difference frequency between said first and second operating frequencies,

phase splitter means connected to said input means and to said first and second oscillator means providing said first oscillator means with said video information signal as an input and said second oscillator means with an input signal equal in amplitude but degrees out of phase with said video information signal whereby said output signal from said mixer means is modulated in accordance with amplitude of said video information signal.

8. In a display system according to claim 6 wherein said monostable multivibrator comprises:

first transistor means being normally non-conductive,

second transistor means being normally conductive,

first circuit means connected to the emitters of said first and second transistor means preventing said first and second transistor means from ever becoming fully conducting or non-conducting,

second circuit means including a capacitor connected between the collector of said first transistor means and the base of said second transistor means switching the conductive states of said first and second transistor means when a pulse is applied to the base of said first transistor means,

first means charging said capacitor in a first direction after said first transistor means becomes conductive causing said first transistor means to become nonconductive after a predetermined time,

second means charging said capacitor in the second direction after said first transistor means is switched to the non-conductive state causing the monostable multivibrator to become stable rapidly.

9. In a display system according to claim 7 wherein said monostable multivibrator comprises:

first transistor means being normal-1y non-conductive,

second transistor means being normally conductive,

first circuit means connected to the emitters of said first and second transistor means preventing said first and second transistor means from ever becoming fully conducting or non-conducting,

second circuit means including a capacitor connected between the collector of said first transistor means and the base of said second transistor means switching the conductive states of said first and second transistor means when a pulse is applied to the base of said first transistor means,

first means charging said capacitor in a first direction after said first transistor means becomes conductive causing said first transistor means to becone nonconductive after a predetermined time,

second means charging said capacitor in the second direction after said first transistor means is switched to the non-conductive state causing the monostable multivibrator to become stable rapidly.

10. A display system for converting video information into variations in tonal shades on the face of a cathode ray tube, comprising in combination:

input means providing a video information signal,

frequency modulator means connected to said input means modulating a carrier frequency in accordance wit-h amplitude variations in said video information signal,

wave shaper means connected to said frequency modulator means providing a train of pulses having a frequency proportional to the frequency of said modulated carrier frequency,

m-onostable multivibrator means connected to said Wave shaper means providing a pulse of constant Width and amplitude in response to each pulse in said train of pulses,

cathode ray tube means,

means connecting said monostable multivibrator means to the intensifier grid of said cathode ray tube causing the electron beam of said cathode ray tube means to be turned on in response to each of said pulses from monostable multivib-ra-tor means,

sweep circuit means connected to said cathode ray tube means for synchronously sweeping said electron beam whereby variations in tonal shades are displayed on the face of said cathode ray tube means in accordance with variations in amplitude of said video information signal.

References Cited by the Examiner 5 UNITED STATES PATENTS 'Reiches 315-30 Forbes 178-7.5

Schlesinger 178-75 Goldberg 178-6.7 Mullin 1786.7

Ernst 178-5.2

Rumble 30788.5

Heyning 30788.5

15 DAVID G. REDINBAUG'H, Primary Examiner.

J. MCHUGH, Assistant Examiner. 

1. IN A DISPLAY SYSTEM: FIRST MEANS FREQUENCY MODULATING A CARRIER FREQUENCY IN ACCORDANCE WITH AMPLITUDE VARIATIONS IN AN INPUT SIGNAL, SECOND MEANS INCLUDING WAVE SHAPER MEANS AND MULTIVIBRATOR MEANS CONNECTED TO SAID FIRST MEANS GENERATING A PULSE IN RESPONSE TO EACH CYCLE OF SAID CARRIER FREQUENCY, CATHODE RAY TUBE MEANS, THIRD MEANS CONNECTING SAID SECOND MEANS TO THE INTENSIFER GRID OF SAID CATHODE RAY TUBE MEANS WHEREBY THE BEAM OF SAID CATHODE RAY TUBE MEANS IS TURNED ON IN RESPONSE TO EACH OF SAID PULSES. 